The pathogenic treponemes are closely related genetically but they cause clinically distinct diseases: T. pallidum subsp. pallidum causes venereal syphilis, with frequent invasion of the central nervous system and infection of the fetus; T. pallidum subsp. pertenue and T. pallidum subsp. endemicum cause the nonvenereal yaws and bejel, respectively, which do not invade the CNS or infect the fetus in utero. T. paraluiscuniculi causes venereal disease in rabbits but is not infectious for humans. The old syphilis literature tells us that infection-derived immunity is complete for homologous strains but is partial or nonexistent between subspecies or species of pathogenic treponemes. Therefore, any antigenic epitopes shared with other subspecies or species of Treponema are unlikely to be protective. We propose to exploit those genetic and antigenic differences as a tool to identify subspecies- and species-unique genes and molecules that are most likely to play a role in immunity and pathogenesis. New molecules will be identified by immunological selection of antigens from T.p. pertenue and T. paraluiscuniculi expression libraries using specific antisera (Aim 1). Genetic and antigenic differences have already been identified within the tpr gene family, and these genes/antigens will be further explored in the non-pallidum treponemes (Aim 2). For selected genes, expression will be examined by quantitative RT-PCR (Aim 3). Purified recombinant molecules will be used for immunological studies defining species- or subspecies-specificity and development of immunity during infection (Aim 4); ability to induce protective immunity against homologous and heterologous challenge will be examined (Aim 5). This approach will lead to the identification of molecules that are critical to the pathogenesis of, and protective immunity to, treponemal infections.